Ongoing advances in optical fiber and fiberoptic cable development are resulting in further reductions in both fiber attenuation and excess attenuation caused by cabling processes and environmentally-induced microbending. State-of-the-art fibers are evolving such that multi-mode graded-index fibers exhibit attenuation values as low as 0.5 dB/Km at 1.3 microns. With such greatly improved fibers conventional measurement systems need to be improved to provide for highly precise and accurate measurements.
The parameter of cable attenuation together with the maximum change in attenuation over the operating environmental conditions may be the primary factor that determines the maximum allowable cable length. For example, given as a design parameter a 30 dB cable loss margin, a cable with a maximum attenuation of 0.5.+-.0.04 dB/Km at 1.3 microns can be from 55.5-65.2 Km long, neglecting splices. The shortest length, 55.5 Km would be chosen because of the .+-. uncertainty. For comparison, if the uncertainty was .+-.0.125 dB/Km then the same cable would be 0.5.+-.0.125 dB/Km at 1.3 microns. For the 30 dB loss margin, the system length would range from 48-80 Km and the shortest length, 48 Km would have to be used to provide for determinable reliability. Now, let the design allow for a cable loss margin of 50 dB as in the case with some laser systems. Given a 0.5 dB/Km cable, the system length for a .+-.0.04 dB/Km uncertainty is 92.6 Km while for a .+-.0.125 dB/Km uncertainty, the length is reduced to 80 Km. Summarizing the importance of measurement precision, a large measurement uncertainty ultimately results in a substantial reduction in system range. In like manner when losses or other parameter changes are caused by an expected range of pressure, temperature or strain fluctuations, an overall system length reduction and other adjustments need be made in order to have a satisfactorily operating system. These parameters, when precisely predetermined, greatly aid a designer and ultimately produce the most efficient, and, therefore, cost effective, systems.
Thus, there is a continuing need in the state-of-the-art for an apparatus which makes accurate and precise attenuation measurements on both cabled and uncabled multi-mode, graded-index fibers and simultaneously includes features of two measurement systems, the optical time domain reflectometer (OTDR) and the throughput loss system by integrating them via a fiberoptic coupler.